Semiconductor device manufacturing places high demands on the lithographic processes that are the means by which the submicron features are generated. New geometries and ever-shrinking dimensions of microelectronic devices dictate increased resist performance in terms of ability to produce higher resolution features with higher aspect ratios and the ability to image over topography.
The dramatic increase in performance and cost reduction in the electronics industry can be attributed in large parts to innovations in the field of optical lithography. Optical projection step-and-scan machines offer significantly higher throughput as compared to other patterning techniques and are the overwhelming choice for patterning advanced integrated circuits in manufacturing. The minimum printable feature size (resolution) has also decreased by orders of magnitude enabling more complex and higher density circuitry.
In optical projection lithography, resolution is typically governed by the equation.W=k1λ/NA
where W is the minimum printable feature size, λ is the exposure wavelength, NA is the numerical aperture and k1 is constant representing the lithography process. To improve resolution, exposure wavelength has steadily decreased from mercury lamp G-line (436 nm) to H-line (405 nm) to I-line (365 nm) to deep UV (248 nm and 193 nm) while the numerical aperture of the optics has steadily increased.
Advances in resist materials, processes and mask making, and innovations such as reticle enhancement techniques (RET) and use of off-axis illumination (OAI) have also reduced k1 to a point where its values in the range of 0.30-0.45 are typical in manufacturing today. Considering 0.25 is the theoretical limit of diffraction optics this is truly remarkable. Future gains in resolution will be achieved through enhancements such as immersion lithography. In immersion lithography, a fluid with an index of refraction greater than air (n=1.0) is introduced between the imaging surface and the last lens element of the imaging optics. This enables the numerical aperture of optical lithography systems to exceed 1.0 and possibly approach values close to the index of refraction of the immersion fluid. Using water as the immersion fluid, lithography systems with numerical apertures as high as 1.35 may be possible.
Further reduction in wavelength is yet another method to improve resolution. Possible options include F2 excimer laser (157 nm) and extreme UV (13 nm) light. However, as wavelength becomes shorter, the light source becomes more complex and expensive. In addition, the technological complexity with imaging materials, processes, optics and masks required to support imaging using the shorter wavelength also increases dramatically. It is conceivable that the cost of migrating to a new wavelength may be prohibitive and unjustifiable from a fiscal perspective. Thus, it is conceivable that optical lithography using ArF excimer laser (193 nm) may be the only cost-effective optical lithography option available for some time to come. To extend ArF lithography for sub-wavelength patterning beyond the 90 nm half-pitch, process and material innovations will be crucial.
Near field imaging has been reported in recent years as a method to print features smaller than the diffraction limit of optical lithography. Researchers have reported printing of patterns as small as λ/40 where λ is the wavelength of the incident radiation. Most optical near field lithography applications rely on placing an image transducer such as a conformal light-coupling mask or a solid immersion lens in very close proximity to the imaging photoresist layer while it is being irradiated. As the imaging radiation interacts with the image transducer it is altered. If the image transducer is of very small dimension then the alteration is only experienced within a very short distance from the transducer. Thus such effects are often classified as Near Field effects. The physical phenomenon behind the image alteration can be complex. Near field effect, phase shifting and evanescent wave effects are some of the complex explanations that have been provided. All near field imaging references in literature propose the utilization of a detached image modifying layer which is placed in close proximity to the imaging resist. See, for example, M. Paulus, B. Michel, O. Martin, “Near-field distribution in light-coupling masks for contact lithography”, J. Vac. Sci. Technol. B, 17, 6, p 3314-3317 (1999); T. Milster: T. Chen, D. Nam, E. Schlesinger, “Maskless Lithography with Solid Immersion Lens Nano Probes”, Proc. SPIE, 5567: p 545 (2004); J. Rogers, K. Paul, R. Jackman: G. Whitesides, “Using an elastomeric phase mask for sub-100 nm photolithography in the optical near field”, Appl. Phys. Lett., 70, 20, p 2658-2660 (1997); H. Schmid, Hans Biebuyck, B. Michel, O. Martin, “Light-coupling masks for lensless, sub-wavelength optical lithography”, Appl. Phys. Lett., 72, 19, p 2379-2381 (1998); T. Ito, M. Ogino, T. Yamada, Y. Inao, T. Yamaguchi, N. Mizutani, R. Kuroda “Fabrication of sub-100 nm Patterns using Near-field Mask Lithography with Ultra-thin Resist Process”, J. Photopolym. Sci. Technol., 18, 3, p 435-441 (2005); and Ong et al (J. Vac. Sci. Technol, B, v1, 4, 1983), the disclosures of which are incorporated by reference herein.
There are several ways in which a single resist layer can be patterned using multiple exposures. Such methods typically utilize one resist and two masks. The two exposures can be complementary or may be overlapping. In the case of overlapping exposures, the resist is exposed with the first mask with a dose which is the same or smaller than the required dose to print an image, then the second exposure is done with second mask with the same or smaller than the required dose. With this one resist approach, some double exposures are used to enhance resolution such as to print gates as described in U.S. Pat. No. 6,586,168, the disclosure of which is incorporated by reference herein, some are used to correct features which are hard to correct with OPC such as to fix line end shortening as described in U.S. Pat. No. 6,566,019, the disclosure of which is incorporated by reference herein. When two resists are introduced in the double exposure approach, prior arts do not use the patterned top layer to modify the images of the bottom resist layer. The two layers are patterned independently to form additive features such as the applications for dual damascene processes as described in U.S. Pat. Nos. 6,242,344 and 5,877,076, the disclosures of which are incorporated by reference herein, or to form subtractive features such as the application to contact holes with packing and unpacking scheme as described in U.S. Pat. No. 6,664,011, the disclosure of which is incorporated by reference herein.
Accordingly, it is a purpose of the invention to have high resolution imaging of semiconductor features.
It is another purpose of the present invention to have high resolution imaging of semiconductor features in which an image modifying layer is utilized.
It is yet another purpose of the present invention to have high resolution imaging of semiconductor features in which the image modifying layer conforms to the topography of the underlying surface.
These and other purposes of the invention will become more apparent after referring to the following description of the invention in conjunction with the accompanying drawings,